There are devices known in the related art that measure a partial discharge that may occur in a rotating electric machine engaged in operation and provide a diagnosis of the insulation conditions in the rotating electric machine based upon the measurement results (see, for instance, patent literature 1). Such a device is equipped with a first detector disposed in close proximity to a stator winding of the rotating electric machine, which detects a first high-frequency signal generated due to a partial discharge occurring at the stator winding. It is further equipped with a second detector disposed inside a frame housing the stator winding and connected in series to the first detector, which detects a second high-frequency signal propagated from the first high-frequency signal. Then, the device determines the insulation conditions of the rotating electric machine, i.e., the state of health of the rotating electric machine, by analyzing components of a specific frequency band of the first and second high-frequency signals.